1. Technical Field
The present invention relates to integrated circuits and, more specifically, to current-controlled pulse width modulation (PWM) circuits and class-D amplifiers comprising the same.
2. Discussion of the Related Art
PWM is a circuit technique for carrying information by modulating a pulse width of a periodic signal. A typical circuit for generating a PWM signal employs a switching and/or control regulator. A PWM control regulator is used to vary pulse widths for operations with fixed frequencies and variable duty ratios. In addition, outputs from PWM circuits are usually employed to conduct switching operations of pass transistors having very small resistances.
Referring to FIG. 1, a conventional PWM circuit 100 includes a pyramidal wave generator 110, a first comparator 120, a low pass filter (LPF) 130, and a second comparator 140. The pyramidal wave generator 110 creates a signal with a pyramidal waveform and transfers it to the first comparator 120. The first comparator 120 generates a PWM output signal PWMOUT by comparing the output signal of the pyramidal wave generator 110 with an output signal of the second comparator 140. The second comparator 140 transfers an output, made by comparing an output of the LPF 130 with an audio signal, to the first comparator 120.
The PWM circuit 100 includes a feedback loop for the PWM output signal PWMOUT by way of the LPF 130. This feedback loop, however, may result in signal degradation as the PWM output signal PWMOUT passes through the LPF 130 or from noises induced by the LPF 130. Such noises may deteriorate the PWM output signal PWMOUT when short pulses of noise coincide with the signal as it passes through the LPF 130 or degrade the signal such that it is not observable. Furthermore, as the PWM output signal PWMOUT is formed (based on the output of the pyramidal wave generator 110), its frequency becomes fixed at a constant value that is typically lower than the frequencies required for systems using higher frequencies.
Therefore, there is a need for a PWM circuit that can generate a PWM output signal with a variable frequency.